1. Field of the Invention
The present invention generally relates to an injection-locked frequency divider, and more particularly, to an injection-locked frequency divider, based on a mixer and a voltage control oscillator, having a wide injection-locking range and a low phase noise.
2. Description of Related Art
The rapid development of wireless communication, not only drastically improves quality of human life, but also brings huge economic profit. Up to now, one may instantly communicate and share information with others simply by a notebook, a personal digital assistant, or a cellular phone. In a typical wireless communication system, a frequency synthesizer is a critical component for generating carrier wave signals. The frequency synthesizer usually includes a frequency divider for dividing a frequency of an input signal, so as to generate a signal with desired frequency. Such frequency dividers are also widely used in multiplexers, phase locked loops, and clock pulse generators.
Frequency dividers are often classified into digital frequency dividers and analog frequency dividers. Digital frequency dividers include common mode logic frequency dividers and dynamic logic frequency dividers. Analog frequency dividers include Miller dividers and injection-locked frequency dividers. Typically, a frequency divider often consumes much power when operating under a high frequency, so as to decrease the operation efficiency of the system. Comparatively, an injection-locked frequency divider, in an RF communication system, usually having a higher operation frequency and lower power consumption than other frequency dividers, is often selected.
FIG. 1 is a circuit diagram illustrating a conventional injection-locked frequency divider. Referring to FIG. 1, there is shown a conventional injection-locked frequency divider 100 including a signal injection unit 110, and an LC resonance tank 120. The signal injection unit 110 includes a P type transistor P1, for receiving an injection signal Vinj having a frequency fi. The LC resonance tank 120 includes the inductances I1, I2, and the variable capacitors Cf1, Cf2. In the injection-locked frequency divider 100, a signal Vtune is introduced to control voltage differences over the variable capacitors Cf1 and Cf2, so as to adjust an oscillation frequency fo of a frequency dividing signal S1. When the oscillation frequency fo approximates to a half of the frequency fi of the injection signal Vinj, the injection-locked frequency divider 100 locks and outputs the frequency dividing signal S1 with a frequency of fi/2 via nodes A and B. The injection-locked frequency divider 100 further includes P type transistors P2 and P3. The P type transistors P2 and P3 are cross coupled to generate a negative resistance for eliminating an equivalent resistance generated by the LC resonance tank 120.
However, when the oscillation frequency fo is too much different from a half of the injection signal (fi/2), the injection-locked frequency divider 100 is then incapable of locking the frequency of the frequency dividing signal S1. Generally, a ratio LR of a locking range is used for describing that a highest frequency fIH that can be locked minus a lowest frequency fIL that can be locked, and then is divided by two times of the oscillation frequency fo of the LC resonance tank 120, which can be represented by an equation as LR=(fIH−fIL)/(2×fo). Although the conventional injection-locked frequency divider 100 is adapted for operation under a very high frequency, unfortunately its locking range is too narrow, so that the injection-locked frequency divider 100 has too narrow a range for frequency dividing. Even though the variable capacitors Cf1, Cf2 are employed for adjusting the oscillation frequency, the locking range can not be effectively improved.
As such, to further modify the conventional injection-locked frequency divider and providing a solution of the narrow locking range thereof become an important concern of the research of injection-locked frequency dividers.